1. Field of the Invention
The field of the present invention is that of turbomachines and more particularly that of combustion chambers for these turbomachines.
2. Description of the Related Art
The combustion chamber of a gas turbine engine receives compressed air that comes from a high-pressure compressor arranged upstream, and delivers, downstream, a gas which has been heated up by the combustion of a fuel mixed with this compressed air. The chamber is generally of annular type and is housed inside an engine case, downstream of the diffuser the function of which is, by slowing down the stream of air, to convert the energy of the compression into a form that is compatible with the operation of the combustion engine and to orient the stream of compressed air leaving the compressor. It also comprises an inner wall and an outer wall between them delimiting a combustion zone. In its upstream part the chamber comprises a transverse chamber end wall in which openings are formed, each opening being equipped with a system for supplying carbureted air. Such a system is supplied with fuel from a liquid fuel injector and generally comprises concentric annular cascades which generate a swirling air stream encouraging the air to mix with the sheet of atomized fuel. The combustion chamber ends downstream in an opening which opens onto a turbine nozzle and, more generally, onto the turbine module of the turbomachine.
The air from the diffuser enters a zone surrounding the combustion chamber and some of it flows along the outer and inner walls thereof while the rest enters the combustion chamber and plays a part in burning the air-fuel mixture in a combustion zone. Schematically speaking, the combustion zone is split into two parts: a primary zone situated immediately downstream of the chamber end wall and in which the mixture is burnt, in near-stoichiometric proportions thanks to an inlet of air known as the primary air inlet, and a secondary part or dilution zone, situated further downstream, in which the gases are mixed with additional cooling air that enters via holes known as dilution holes.
In the prior art, protection, in the form of sectorized deflectors, lines the inside of the chamber end wall and has the role of protecting it from the intense radiation produced in the primary combustion zone. Air is therefore introduced via orifices made in the chamber end wall behind deflectors in order to cool them. This air flows along the rear face of the deflectors and is then guided to form a film along the interior face of the outer and inner walls of the chamber.
These deflectors are subjected to very high temperatures and, in order not to become burnt during use, they need a large quantity of cooling air, and this detracts from the efficiency of the chamber. It would therefore be desirable to dispense with the deflector, and this would also have significant concomitant advantages: because of the mass of metal it constitutes, the cooling-air consumption is greater than the amount that would be needed for cooling the chamber end wall alone. There would therefore be an advantageous saving on flow rate into the bargain.
To this end, solutions have been conceived of for cooling the chamber end wall without fitting a deflector. One solution that has been put forward is to cool the chamber end wall using multiple perforations and to orient the air stream that passes through these perforations so that it sweeps over the inside of the chamber end wall. This solution is notably described in Patent Application FR 2 856 467 filed in the name of the applicant company. It proposes making cylindrical perforations in the chamber end wall and inclining these perforations by orienting them in such a way that the air streams are increasingly steeply inclined nearer the axis of the chamber. The inclinations described are between 5 and 60°.
While this solution is well suited to an engine in which the compressor is of the axial type, i.e. an engine the diffuser of which is positioned along the axis of the injectors of the combustion chamber, it is not optimal for a turbomachine that has a centrifugal compressor. This is because these engines, which are usually small in size, have the diffuser situated at the periphery of the zone surrounding the combustion chamber and the outlet air is oriented axially, on the outer side of the combustion chamber. There is a risk that the outer wall will therefore be adequately cooled but, on the other hand, that an inner wall will be insufficiently cooled and could become burnt. An increase in the cooling flow rate to counter this phenomenon would impair the efficiency of the chamber and be accompanied by the production of unburnt species such as carbon monoxide CO.
Moreover, this solution has the disadvantage of greater difficulty in defining the cooling circuit during the engine design phase. This is because it is necessary to wait for the detailed engine design phase, with an engine cycle that is already stabilized, before a meaningful characterization of the aerodynamics of the airflow leaving the diffuser becomes available so that the definitive drilling pattern can be optimized. Demanding computation methods have therefore to be used in order to obtain the definitive solution.